The present invention is related to fine denier polyolefin fibers. In particular, the invention is related to fine denier polyolefin fibers obtained by splitting multicomponent polyolefin fibers and to fabrics made from such fine fibers.
Filtration processes are used to separate compounds of one phase from a fluid stream of another phase by passing the fluid stream through filtration media, or septum, which traps the entrained or suspended matter. The fluid stream may be either a liquid stream containing a solid particulate or a gas stream containing a liquid or solid aerosol. Properties considered in selecting a particular filtration media include the ability of the media to retain particulates to be filtered out of the fluid, chemical resistance, physical strength to withstand filtering conditions, and cost.
Common filtration media include fabrics formed of natural, synthetic, metallic and glass fibers. For use in corrosive environments, filters are typically formed of fibers having chemical resistance. Examples of polymers having chemical resistance include polyolefins, such as polyethylene and polypropylene, and fluoropolymers, such as polytetrafluoroethylene. For example, polypropylene fabrics are beneficial for use as septum in a wide range of applications because such fabrics are economical, insensitive to moisture, have adequate tensile properties, are able to retain an electrical charge, and have superior chemical resistance. For a more detailed discussion of various filtration applications employing polypropylene nonwoven fabric, reference is made to U.S. Pat. No. 5,586,997 directed bag filters; U.S. Pat. No. 5,795,369 directed to mist eliminators; and U.S. Pat. Nos. 5,597,645 and 5,792,242, both directed to electret filters. See also U.S. Pat. No. 4,874,399, reporting additional benefits of a polypropylene blend in electret filter applications.
A wide range of fabric constructions can be used in filtration media, including woven, knit, and nonwoven fabrics. For example, meltblown and melt spun nonwoven webs have been used as septum. Exemplary melt spun webs include carded fiber webs, air-laid fiber webs, wet-laid fiber webs and spunbond fiber webs.
Fine denier fibers in filtration media can provide benefits in the filtration of extremely small particulates. Fine denier fibers may be used to produce fabrics having smaller pore sizes, thus allowing smaller particulates to be filtered from a fluid stream. In addition, fine denier fibers can provide a greater surface area per unit weight of fiber, which can be beneficial in filtration applications.
Meltblown technology is one avenue by which to produce fabric from fine denier filaments. Fine denier meltblown webs have been widely employed as filter media because the densely packed fibers of these webs are conducive for providing high filter efficiency. However, meltblown webs typically do not have good physical strength, primarily because less orientation is imparted to the polymer during processing and lower molecular weight resins are employed. Thus, in general, meltblown filter media are laminated to at least one separate, self-supporting layer, which adds cost and complexity to the manufacturing process.
Melt extrusion processes can provide higher strength fibers than meltblown fibers. However, it is difficult to produce fine denier fibers, in particular fibers of 2 denier or less, using conventional melt extrusion processes. Therefore, while filter media produced from nonwoven webs of coarser fibers, such as spunbond and staple fiber webs, have been used in filtration applications such as stove hood filters, they have not been used as filter media for fine particles.
One avenue by which to overcome this difficulty in melt extrusion is to split multicomponent continuous filament or staple fiber into fine denier filaments, or microfilaments, in which each fine denier filament has only one polymer component. Multicomponent fibers, also referred to as composite fibers, may be split into fine fibers comprised of the respective components, if the composite fiber is formed from polymers which are incompatible in some respect. The single composite filament thus becomes a bundle of individual component microfilaments. See, for example, U.S. Pat. Nos. 5,783,503 and 5,759,926, reporting splittable multicomponent fibers containing polypropylene, such as splittable polyester/polypropylene and nylon/polypropylene fibers.
A number of processes are known for separating the fine denier filaments from multicomponent fibers. The particular process employed depends upon the specific combination of components comprising the fiber, as well as their configuration. One common process by which to divide a multicomponent fiber involves mechanically working the fiber. Methods commonly employed to work the fiber include drawing on godet rolls, beating or carding. It is also known that fabric formation processes such as needle punching or hydroentangling may supply sufficient energy to a multicomponent fiber to effect separation. When mechanical action is used to separate multicomponent fibers, the fiber components must be selected to bond poorly with each other to facilitate subsequent separation. In that vein, conventional opinion has been that the polymer components must differ from each other significantly to ensure minimal interfilamentary bonding. It is for this reason that polymers having disparate chemistries, i.e., from different chemical families, have been chosen as components for mechanically dissociable composite fibers to date.
However, the use of such disparate chemistries is problematic, as polymers from different chemical families generally have physical properties which differ significantly, such as chemical resistance. As an example, when filtering corrosive fluids using fabric formed from polyester/polypropylene multicomponent fiber, the polyester component will readily degrade, while the polypropylene component will withstand the chemical attack.
Based on the foregoing, although a number of methods for splitting multicomponent fibers containing polyolefin components to obtain fine denier filaments are known, there is still need for improvement.
The present invention provides splittable multicomponent polyolefin fibers and fiber bundles which include a plurality of fine denier polyolefin filaments having many varied applications in the textile and industrial sector. The fibers can exhibit many advantageous properties, such as a high surface area per weight, chemical resistance, a soft silk-like hand, and the like. The present invention further provides fabrics formed of the multicomponent fibers and fiber bundles, as well as an economical process by which to produce fine denier polyolefin filaments.
In particular, the invention provides mechanically divisible or splittable fibers formed of polyolefin components. The fibers can have a variety of configurations, including pie/wedge fibers, segmented round fibers, segmented oval fibers, segmented rectangular fibers, segmented ribbon fibers, and segmented multilobal fibers. Further, the mechanically splittable multicomponent fibers can be in the form of continuous filaments, staple fibers, or meltblown fibers. The splittable fibers may be dissociated by a variety of mechanical actions, such as impinging with high pressure water, carding, crimping, drawing, and the like.
In one particularly advantageous aspect of the invention, the divisible multicomponent fiber includes at least one polyolefin component containing branched alkyl radicals, advantageously poly(4-methyl-1-pentene) (PMP), and at least one polyolefin component containing straight-chain alkyl radicals, advantageously polypropylene (PP). The polymer components are dissociable by mechanical means to form a bundle of fine denier polyolefin fibers. A particularly advantageous embodiment is a splittable multicomponent fiber formed of poly(4-methyl-1-pentene) and polypropylene in a pie/wedge configuration.
The instant invention also provides a fiber bundle which includes a plurality of dissociated polyolefin microfibers of different polyolefin compositions. Specifically the fiber bundle includes a plurality of branched alkyl polyolefin microfilaments, advantageously poly(4-methyl-1-pentene) microfilaments, and straight-chain alkyl polyolefin microfilaments, advantageously polypropylene. In general, the microfilaments of the present invention range in size from 0.05 to 1.5 denier.
The multicomponent fibers can be formed into a variety of textile structures, including nonwoven webs, either prior to or after fiber dissociation. Fabrics made using the fine denier fibers of the present invention are both economical to produce and behave in important ways as fabrics made entirely of polyolefin. As noted previously, earlier fabrics containing mechanically splittable composite filaments were based on disparate component chemistries. A typical conventional fabric produced from mechanically splittable composite fibers includes polypropylene (PP) and polyethylene terephthalate (PET) microfilaments. As noted previously, PET/PP fabrics are not recommended for use as filters for in corrosive environments, because the PET microfilaments degrade, thereby destroying filtration properties. In addition to loss of filtration performance of the dissolved PET fiber, the filtered stream can be contaminated with the PET decomposition products. In contrast, a filter entirely from fine denier polyolefin fibers, such as poly(4-methyl-1-pentene) and polypropylene, would be expected to withstand a broad range of chemical attack for an extended period of time.
However, previous attempts to overcome this difficulty by making mechanically splittable fibers from polyolefins have failed, because most polyolefins have too high an affinity for each other to allow the segments to be split easily. Surprisingly, the inventors have found that a branched alkyl polyolefin polymer, advantageously poly(4-methyl-1-pentene), can be made into a readily splittable segmented melt spun fiber with straight-chain alkyl polyolefins such as polypropylene. The resulting composite fiber has desirable chemical resistance and tensile properties in comparison to comparable fibers produced by melt blowing.
Another aspect of the invention teaches fabrics formed from mechanically splittable multicomponent fibers formed from branched alkyl and straight-chain alkyl polyolefin components, as well as the methods by which to produce such fabrics. In this aspect of the invention, the multicomponent fibers can be divided into microfilaments prior to, during, or following fabric formation. Fabrics of the present invention may generally be formed by weaving, knitting, or nonwoven processes. Advantageously the fabric is a dry-laid nonwoven fabric formed from the multicomponent fibers of the present invention. Another advantageous fabric is a dry-laid nonwoven fabric bonded by hydroentangling.
Products comprising the fabrics of the present invention provide further advantageous embodiments. Particularly preferred products include filtration media, including bag filters, electret filters, and mist eliminators. Filtration media for severe service conditions, in particular corrosive environments, is also provided.
By providing fiber bundles comprised entirely of fine denier polyolefin filaments, the present invention permits soft fabrics having a high degree of coverage to be economically produced. In specific, the multiconstituent fibers of the present invention allow the production of fabrics containing fine denier polyolefin filaments for use in filtration, particularly the filtration of corrosive materials.